...you were not been able to provide us...
Just to be clear, this is a fan based News and Discussion site. It is in no way run by Microsoft, Lumia or any developer.
We are not able to provide an OS or Firmware. It's like asking your cousin who uses Windows Phone to fix it for you. He may want to, but is not in the position to do so.
In terms of your files and data on your phone. Pin lock the screen. That helps prevent Thieves from accessing anything.
If it is a significant partner snooping on your phone, change the pin and never give them the new one. If they persist, get rid of them. They should not be going through your phone.
If it is a friend, get a better class of friend. If I handed a friend my phone and they started poking at all my emails, texts, banking etc. I would smack their hand as hard as I could and tell them, "Don't touch. Baby must learn that fire will burn their little hand."
Honestly, in my 52 years of life it has never been an issue for me that someone I know has snooped on my phone. Take control of this issue yourself.
If this is just to complain about something... Carry on, but please provide a scenario on what casual intrusions on your device you need to prevent.
App locks do not provide against outside breaches any better than the pin lock on the main screen.